This invention relates to an improved process for preparation of bis-(2-pyridyl-1-oxide) disulfide by oxidation of a solution of 2-mercaptopyridine-1-oxide, prepared via the peracetic acid route, with hypochlorous acid under controlled pH conditions.
U.S. Pat. No. 2,742,476 discloses generally the preparation of bis-(2-pyridyl-1-oxide) disulfide, also known as 2,2'-dithiodipyridine-1-1'-dioxide, hereinafter sometimes referred to as "the disulfide", by the general process in which a 2-mercaptopyridine-1-oxide is oxidized with an oxidizing agent of the peroxide type. U.S. Pat. No. 3,759,932 discloses the preparation of bis(2-pyridyl) disulfide by an integrated in situ route wherein the mercaptopyridine intermediate is oxidized without isolation with hydrogen peroxide. In our prior co-pending application Ser. No. 466,328, now U.S. Pat. No. 3,892,760 it was disclosed that in an integrated in situ process where permaleic acid was utilized in the initial oxidation step to convert 2-chloropyridine to 2-chloropyridine oxide followed by a subsequent peroxide oxidation of an intermediate mercaptopyridine-1-oxide, a preliminary pH adjustment to pH 4-6 was required in order to avoid certain precipitates caused by the initial use of permaleic acid.
It is also known that chlorine may be utilized in an integrated in situ process to oxidize 2-mercaptopyridine-1-oxide and its salts to the disulfide. The use of chlorine instead of peroxide is extremely desirable due to the inherent dangers implied in the handling of peroxides, due to the capital outlays for special equipment required to handle the latter safely and due to the cost differential between peroxide and chlorine. However, chlorine has been found to be unsuitable for the final oxidation in an integrated process utilizing permaleic acid in the initial oxidation of 2-chloropyridine to 2-chloropyridine-1-oxide. In such a system, the chlorine substitutes across the double bond of the permaleic acid and produces insoluble chlorinated by-products which preceipitate with the disulfide, complicate recovery and reduce yields.
On the other hand where peracetic acid is utilized as the initial oxidant to convert 2-chloropyridine to 2-chloropyridine-1-oxide in an integrated in situ system, chlorine has been utilized as the oxidant to convert the intermediate 2-mercaptopyridine-1-oxide to the disulfide. In prior processes utilizing chlorine, large excesses of chlorine were utilized in order to obtain yields which were barely adequate. Thus, chlorine was found to be a less satisfactory oxidant than peroxide.
We have now found that chlorine efficiency and yields may advantageously increase by chlorinating a solution of 2-mercaptopyridine-1-oxide under pH conditions conducive to the in situ formation of hypochlorous acid, namely at or to a pH in the range of 4 to 8 in the presence of a base selected from the group consisting of an alkali metal hydroxide, carbonate, or bicarbonate.